The field of the invention is nuclear magnetic resonance imaging (MRI) methods and systems. More particularly, the invention relates to the removal of artifacts in MRI images produced by changes in the polarizing magnetic field during data acquisition.
When a substance, such as human tissue, is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with the polarizing field B0, but instead precess about it in random order at a characteristic Larmor frequency, which is determined by the gyromagnetic constant xcex3 of the spins and the polarizing field B0. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment Mz may be rotated or xe2x80x9ctippedxe2x80x9d into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins, and after the excitation field B1 is terminated, a nuclear magnetic resonance (NMR) signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of separate measurement cycles (referred to as xe2x80x9cviewsxe2x80x9d) in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well-known reconstruction techniques.
A well-known problem with MRI systems is that variations occur in the strength of the polarizing field B0. Such variations affect acquired images in two ways. First, changes in the polarizing field B0 cause corresponding changes in the phase of the acquired NMR signals. Such spurious phase changes appear in the acquired NMR signals, or xe2x80x9ck-space dataxe2x80x9d, and result in the ghosting or blurring of artifacts in images reconstructed using Fourier transformation methods. Since the spurious phase changes accumulate continuously between RF excitation and data acquisition, the artifacts are particularly troublesome with gradient recalled echo pulse sequences having a long echo time TE. Changes in the polarizing field B0 can also cause apparent spatial shifts along the frequency encoding (i.e. readout) gradient direction.
The second deleterious effect of changes in the polarizing field B0 occurs when slice selection techniques are used in the pulse sequence. The change in the polarizing field B0 shifts the location of the excited slice by an amount equal to the change in Larmor frequency divided by the bandwidth of the selective RF excitation pulse. For example, if the polarizing field B0 shifts the Larmor frequency by 20 Hz and the selective RF excitation pulse has a bandwidth of 1000 Hz, the excited slice will shift 2% from its expected position along the slice select gradient axis. Such shifts can cause amplitude changes in the acquired data.
Many methods are used to control and regulate the polarizing field B0. Most of these methods deal with changing conditions within the scanner itself and are quite. effective. For example, methods for compensating the effects on the polarizing field B0 due to Eddy currents produced by changing magnetic field gradients are disclosed in U.S. Pat. Nos. 4,698,591; 5,289,127; and 5,770,943.
The strength of the polarizing field B0 can be affected by external events such as the movement of large masses of metal in the vicinity of the scanner. Moving objects, such as cars, trucks, trains and elevators, often change the polarizing field B0 and produce image artifacts.
Two methods have been used to reduce the effects of such disturbances, namely passive methods and active methods. Passive methods include the use of shielding materials around the main magnet as described, for example, in U.S. Pat. No. 4,646,046. Massive amounts of silicon steel sheets are placed around the magnet, which results in an expensive, heavy, and difficult to install system.
Active compensation systems employ a sensor that measures the change in magnetic flux at a location near the scanner and uses this information to compensate the system. Such compensation may include producing a current in a coil that generates an offsetting correction magnetic field. Such methods employ flux sensors as described, for example, in U.S. Pat. No. 5,952,734 or in ESR instruments as disclosed in U.S. Pat. No. 5,488,950. These active methods do not work well when the field disturbance is produced by multiple sources or sources of varying magnitude or location.
Acquiring NMR signals in order to monitor the stability of a magnetic field can be achieved using a dedicated probe that consists of an RF coil and an NMR sample that is placed in a region that is consistent with reliable magnetic field uniformity. Placement of the probe in the shimmed volume of the MRI magnets interferes with the region to be occupied and is subject to operator error. Placement of the probe outside the shimmed volume of the MRI magnets, on the other hand, degrades the quality of the signal from the dedicated NMR probe. What is needed, therefore, is an apparatus and method allowing placement of the dedicated monitoring probe outside the shimmed volume of the magnet while retaining adequate magnetic field uniformity in order to improve the utility and quality of the acquired monitor signals.
In recognition of the foregoing problems and shortcomings, the method of the present invention acquires an image from a subject with an MRI system by performing an imaging pulse sequence with the MRI system in order to acquire image data from which an image is reconstructed. Then, the method performs a plurality of monitoring pulse sequences with the MRI system in order to acquire data from an RF monitoring coil that is located near the subject for compensating for changes in the polarizing field B0 during the acquisition of image data. As a result, monitoring pulse sequences are interleaved with imaging pulse sequences. Finally, the present method shims the polarizing field B0 in the immediate vicinity of the RF monitor coil by producing currents in the shim coils during each monitoring pulse sequence.
Another aspect of the present invention includes a monitor coil assembly comprising an NMR sample, a monitor coil disposed around the sample, and a set of shim coils disposed adjacent to the monitor coil, and a set of shim amplifiers whereby electrical currents are produced in the set of shim coils that shim the polarizing field B0 in the immediate vicinity proximal to the monitor coil.